Fix scrollbar track offset (flutter#106835)

Author: Piinks

packages/flutter/lib/src/widgets/scrollbar.dart

@@ -488,7 +488,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
         x = crossAxisMargin + padding.left;
         y = _thumbOffset;
-        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin);
+        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin + padding.top);
         borderStart = trackOffset + Offset(trackSize.width, 0.0);
         borderEnd = Offset(trackOffset.dx + trackSize.width, trackOffset.dy + _trackExtent);
         break;
@@ -497,7 +497,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(thickness + 2 * crossAxisMargin, _trackExtent);
         x = size.width - thickness - crossAxisMargin - padding.right;
         y = _thumbOffset;
-        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin);
+        trackOffset = Offset(x - crossAxisMargin, mainAxisMargin + padding.top);
         borderStart = trackOffset;
         borderEnd = Offset(trackOffset.dx, trackOffset.dy + _trackExtent);
         break;
@@ -506,7 +506,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
         y = crossAxisMargin + padding.top;
-        trackOffset = Offset(mainAxisMargin, y - crossAxisMargin);
+        trackOffset = Offset(mainAxisMargin + padding.left, y - crossAxisMargin);
         borderStart = trackOffset + Offset(0.0, trackSize.height);
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy + trackSize.height);
         break;
@@ -515,7 +515,7 @@ class ScrollbarPainter extends ChangeNotifier implements CustomPainter {
         trackSize = Size(_trackExtent, thickness + 2 * crossAxisMargin);
         x = _thumbOffset;
         y = size.height - thickness - crossAxisMargin - padding.bottom;
-        trackOffset = Offset(mainAxisMargin, y - crossAxisMargin);
+        trackOffset = Offset(mainAxisMargin + padding.left, y - crossAxisMargin);
         borderStart = trackOffset;
         borderEnd = Offset(trackOffset.dx + _trackExtent, trackOffset.dy);
         break;

packages/flutter/test/widgets/scrollbar_test.dart

@@ -2565,4 +2565,36 @@ void main() {
 
     // Go without throw.
   });
+
+  testWidgets('Track offset respects padding', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/106834
+    final ScrollController scrollController = ScrollController();
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: MediaQuery(
+          data: const MediaQueryData(
+            padding: EdgeInsets.all(50.0),
+          ),
+          child: RawScrollbar(
+            controller: scrollController,
+            minThumbLength: 21,
+            minOverscrollLength: 8,
+            thumbVisibility: true,
+            child: SingleChildScrollView(
+              controller: scrollController,
+              child: const SizedBox(width: 1000.0, height: 50000.0),
+            ),
+          ),
+        )
+      )
+    );
+    await tester.pumpAndSettle();
+    expect(
+      find.byType(RawScrollbar),
+      paints
+        ..rect(rect: const Rect.fromLTRB(744.0, 50.0, 750.0, 550.0)) // track
+        ..rect(rect: const Rect.fromLTRB(744.0, 50.0, 750.0, 71.0))
+    ); // thumb
+  });
 }